If I hold down CTRL and then press D, and then D again without letting go of the CTRL key and pressing the CTRL key again, an error occurs. That happens with Z too, others I've never tried.
I'm not sure it's a bug, but in GIMP you can hold down CTRL and press Z as many times as you want to undo something without an error occuring, so I find it annoying. If it isn't a bug then this is a request for the above to be added to a new version.

